1). Field of the Invention
This invention relates to an x-ray based non-intrusive inspection apparatus and to a method of non-intrusively inspecting an object.
2). Discussion of Related Art
Inspection apparatus are commonly used for non-intrusively inspecting luggage or other closed containers before being loaded into a loading bay of an aircraft. Older generation inspection apparatus relied merely on conventional x-ray technology for non-intrusively inspecting closed containers. More recently, inspection apparatus which rely on computer tomography (CT) scanning technology have also been utilized.
An apparatus that utilizes CT scanning technology typically has a frame and a CT scanning subsystem rotatably mounted to the frame. The CT scanner subsystem has a gantry with an opening through which an object, typically luggage, can pass. An x-ray source is mounted to the gantry and radiates x-rays through the object. X-ray detectors are mounted to the gantry on an opposing side of the opening, so as to detect the x-rays after leaving the object. The x-ray source and detectors revolve together with the gantry around the object. A three-dimensional rendering of the contents of the object can be obtained by revolving the gantry and progressing the object through the x-rays.
FIG. 8 illustrates a conventional x-ray based non-intrusive inspection apparatus. An x-ray source 32 radiates x-rays with an included angle A of approximately 70°. Detectors 34 detect the x-rays after passing through an object. The x-ray source 32 and detectors 34 revolve about a center axis C so that an object can be inspected from different sides.
Shadow lines L1 and L2 can be constructed in space where there is a transition from x-rays to no x-rays, i.e., from detected x-rays to non-detected x-rays. The shadow lines L1 and L2 are tangential to a circle of reconstruction 44 having a center axis that coincides with the center axis C. The object being scanned should pass entirely through the circle of reconstruction 44 so that a three-dimensional representation can be obtained from any portion of the object.
The size of the circle of reconstruction 44 depends on two factors, namely the size of the included angle A, and the distance of the x-ray source 32 from the center axis C. The included angle A can only be increased to approximately 70°, whereafter there is a degradation in the quality of an object that can be resolved because (i) the heel effect of the x-ray source 32 causes degradation of the x-ray spectrum, and (ii) the focal spot seen by one of the detectors 34 becomes bigger. The x-ray source 32, accordingly, has to be placed relatively far from the center axis C to obtain a sufficiently large circle of reconstruction 44 while maintaining the included angle A less than 70°. A large distance from the center axis C to the x-ray source 32, however, results in a large gantry and support frame that may not be suitable for placement in certain confined spaces. A larger gantry also requires larger forces to accelerate and decelerate the gantry. The larger forces, in turn, necessitate the design of a larger, stronger, and heavier support frame.